The present invention relates to an improvement of a water-tight structure for a multicore cable suitably used in, e.g., an electromagnetic pump used for supplying fuel to a vehicle.
An electromagnetic pump used as a vehicle fuel supply pump or the like is often mounted outside the vehicle, particularly, under the floor. The electromagnetic pump tends to be contaminated with water or dirt during vehicle travel. Therefore, a pump housing constituting an outer casing of the pump requires a waterproof joint and the like. This requirement is apparent because an electrical chamber having an excitation coil, a transistor, a printed circuit board, and the like therein is defined in the pump housing.
For this reason, in a conventional electromagnetic pump of this type, a pump housing which accommodates the electrical components described above is constituted by a sealed vessel to assure waterproofness. In this case, a problem is posed at a cable lead portion for connecting electrical components to external devices. More specifically, cables for externally connecting the electrical components arranged in the electrical chamber are led from a cable lead portion arranged at part of the pump housing through a grommet or the like. The grommet can relatively easily achieve waterproofness between housing holes and the cable covering. However, water or moisture inevitably enters into the pump housing at the cable lead portion through the inside of the cable covering. In the cable, inner conductor ends which have the insulation stripped therefrom inside the pump housing are exposed in the electrical chamber, while outer ends of the stripped conductors are connected to terminals of external equipment or connectors. A conductive state is assured through the conductive portions in the stripped portions. An internal pressure of the electrical chamber tends to be changed due to changes in temperature. The internal pressure may serve as a negative pressure with respect to the external pressure. In this case, it is readily understood that water or moisture is drawn into the electrical chamber through the interior portions of the conductors.
In this case, if the cable comprises a single core wire, a critical problem does not occur. However, a single core cable cannot assure the mechanical strength for use as a cable, such as a cable for a vehicle fuel pump, which is subjected to vibrations and bending. Therefore, a multicore cable obtained by twisting a plurality of thin wires is generally used because it has flexibility and tends to be easily bent. In such a multicore cable, however, the shape of a space between thin wires and the shape at an outer portion of the cable form an in-flow path. As a result, such a multicore cable cannot provide a good seal.
The electromagnetic pump described above employs a cable described in U.S. Pat. No. 4,299,544. In this cable, a foamed synthetic resin material is filled or applied to the electrical chamber from which the cable extends. Inner conductor end portions of the cable which are stripped of insulation inside the electrical chamber are sealed with the resin material, and water, salt water, and moisture which tend to enter into the stripped inner end cable portion inside the electrical chamber are shielded, thus constituting a general sealing structure.
According to the conventional sealing structure, however, the resin material filled or applied to the inside of the pump housing has low heat resistance and has poor wettability with an oil. In addition, during filling or application of this resin material, strict conditions such as conditions for contamination of the seal surfaces, a filling amount, an application amount, a dry time, and air accumulation, are required, and sealing properties undesirably vary, thus failing to satisfy sealing reliability. In addition, when such a resin material is utilized, the working processes are complicated to result in cumbersome operations. Moreover, the resin may leak outside the housing to contaminate the outer surface of the housing. The leaking resin component must be removed. Therefore, strong demand has arisen for another sealing countermeasure.
A connecting component such as a hermetic seal may be used at the cable lead portion for leading a cable from the pump housing to perfectly seal the electrical chamber. With this arrangement, the number of components constituting the cable lead portion is increased, and the assembly operation is also complicated. In addition, thermal expansion coefficients of the components must be matched at the cable lead portion, resulting in high cost. In particular, a terminal structure by such a hermetic seal tends to be damaged by a thermal effect or impact and tends to be cracked by vibrations. A component for reinforcing the terminal structure is thus required. As a result, the number of components is further increased, and an increase in cost is unavoidable.
Still another conventional technique is also proposed. A compression force of rubber or the like is utilized to assure water-tightness between the pump housing and the cable covering at the cable lead portion. The outer ends of the extended cable are connected with waterproof connectors or the like so as to assure water-tightness. Entrance of moisture or the like through the inside of the cable can thus be prevented by the waterproof connector. However, such a waterproof connector requires a larger number of components, and its structure is complicated. In addition, complicated assembly is required which results in high cost, thus posing many practical problems. Strong demand has arisen to solve all these problems.